A projection is a way to translate the curved surface of the earth onto a
flat map. The challenge is that with every map projection there is an
amount of distortion because of the spherical nature of the globe cannot
be perfectly represented in a two dimensional map. For a more detailed
discussion of different map projections visit the National Atlas website.

The picture below from The Geographer's Craft Project at University of Colorado at Boulder, illustrates similarities and differences of three different projections. This image powerfully demonstrates the importance of defining the projection, while it may seem like it all the projections are lining up well in Iowa and the Midwest as you move further away from the center of the country the differences in distortion become apparent. The location and size of states like Washington, Maine and Florida change a lot based on projection.